Essentially a remake of 1949's Señor Droopy, but taking in a medieval setting rather than the country of Mexico, One Droopy Knight casts Droopy and his rival Butch as medieval knights out to slay a dragon and win the hand of a beautiful human princess. However, the dragon considers both as nothing more than minor nuisances. Droopy though, takes more chances to slay the dragon, but it thinks Droopy is small and harmless. Meanwhile, Butch is being foiled after being molded into a metal can.